The invention relates generally to thin films and the interference microscope and specifically to techniques for calibrating an optical profilometer and for measuring a thin film surface profile.
Optical profilometry is a non-contact method of measuring the surface characteristics of a thin film sample in three dimensions. Optical profilometry is often preferred to contact methods, such as atomic force microscopy and surface contact profilometry, because the latter are intrinsically less accurate and can destroy features of the sample during measurement.
An optical profilometer is one type of interference microscope (interferometer). An interference microscope generally is used either to measure or to visualize the phase differences between two or more beams of electromagnetic radiation, when directed to a thin film it measures the surface features of the thin film sample under investigation. When the microscope measures the phase differences, it generates an interference pattern which a computer can analyze to derive a surface profile of the sample. The microscope and computer together comprise an interferometer system.
Several beams of the radiation used to measure surface features of a thin film may penetrate slightly beneath the surface of the thin film before they are scattered. This penetration depth changes the distance traveled by a beam and may affect the phase difference between the beam and another beam with which it interferes, creating noise in the interference pattern and decreasing the accuracy of the measurement. The noise becomes more significant when the profilometer is used to measure thin film step heights that fall below 10 nanometers, because at this height the penetration depth is on the same order of magnitude as the step height. If radiation with a smaller wavelength and higher energy is used, the noise becomes even greater because this radiation penetrates even deeper into the thin film. Moreover, the smaller the wavelength, the more dramatically a slight difference in the path traveled by the radiation affects the resulting phase difference as well as the interference pattern.
The penetration depth of the beam introduces additional inaccuracies into the process of calibrating the optical profilometer, especially when the profilometer must be calibrated for taking measurements of thin film step heights in the sub-10 nanometer range. Previously the best technique of calibration for these step heights was to calibrate to a much higher step and then extrapolate blindly to a step that is an order of magnitude lower. Alternately, a contacting measuring method might have been used instead of non-contact optical profilometry.
Unfortunately, the technique of calibrating to a higher step may yield imprecise measurements. Alternately, contacting measuring methods may damage the surface features of a thin film sample. Further, these methods are generally less accurate and may be more costly.
Another drawback of measuring a step height with an optical profilometer arises when the step does not have an identical composition in its upper and next lower levels. In particular, if the upper level of the step comprises one metal with one penetration depth while the next lower level comprises a different metal with a different penetration depth, the step might create even more noise in the interference pattern.
Accordingly, an object of one embodiment of the invention is to provide a technique for calibrating an optical profilometer to measure very small step heights. Another object of an embodiment of the invention is to provide a calibration standard for optical profilometry step height measurements. Another object of an embodiment of the invention is to provide a technique for measuring very small step heights. Another object of an embodiment of the invention is to provide a reference standard for thin film process control.
Briefly described, and in accordance with one embodiment thereof, the invention provides a method of calibrating an interferometer system including measuring the step height of a gold step with the interferometer system. The gold step is in a gold layer of a multilayer thin film which acts as a calibration standard. The multilayer thin film (calibration standard) has an optical flat, a first layer on the surface of the optical flat, a second layer on the first layer, a test layer on a part of the second layer, and a gold layer on the test layer and on a part of the second layer uncovered by the test layer. The test layer has a test layer step, and the gold layer has the gold step over the test layer step. The gold step is equivalent in height to the test layer step and exhibits a lower penetration depth than the test layer step beneath it. The gold step also has a uniform (gold) composition in its upper level and next lower level.
In accordance with another embodiment thereof, the invention provides a calibration standard for optical profilometry step height measurements. The calibration standard has a multilayer structure comprising an optical flat, a first layer on the surface of the optical flat, a second layer on the first layer, a test layer on a part of the second layer, and a gold layer on the test layer and on a part of the second layer uncovered by the test layer. The test layer has a test layer step, and the gold layer has a gold step over the test layer step.
In accordance with another embodiment thereof, the invention provides a method of making a reference standard for a thin film sample with one or more component thin film layers. The method includes depositing a layer of gold over the surface of the thin film sample.
In accordance with another embodiment thereof, the invention provides a reference standard for a thin film sample with one or more component thin film layers. The reference standard has a layer of gold that is measured by the profilometer, but is otherwise essentially the same as the thin film sample.